The present invention relates to a switched-capacity amplifier.
It also relates to switched-capacity filters and to charge-transfer filters comprising such an amplifier.
An amplifier is known which comprises essentially n capacities periodically switched in parallel and in series; the same voltage charges simultaneously the n capacities which are switched in parallel; an amplified voltage is obtained between the end terminals A and B of the n capacities which are switched in series. The periodic switching of the n capacities in parallel and in series is achieved by means of MOS transistors operating as switches. Finally, the n capacities and the transistors are integrated on the same semiconductor substrate which is generally silicon.
The capacities are formed by a technology which comprises a layer of silica covering the substrate and in which are integrated two layers of polycrystalline silicon disposed parallel to the substrate. On the silica layer is deposited a layer of aluminium comprising two parts insulated from each other and each plate of the integrated capacity is formed by a part of the aluminium layer connected to one of the polycrystalline silicon layers.
The problem which arises is that each integrated capacity C comprises a parasite capacity between each of its plates and the substrate. One of the parasite capacities C.sub.P2 is fixed and small in value but the other parasite capacity C.sub.P1 has a high value proportional to the value of the integrated capacity.
The parasite capacities C.sub.P1 cause reduction in the gain on the known switched-capacity amplifier. Moreover, when this amplifier is used as a differential amplifier, the parasite capacities C.sub.P1 cause a considerable increase of the common mode.
It is known to provide compensation for the parasite capacities in the case of a differential amplifier comprising two capacities to be switched. This compensation abolishes the common mode but not the reduction in gain due to the parasite capacities C.sub.P1. Finally, this compensation has the major disadvantage of being only applicable to a given type of amplifier.
It is known from the article appearing in the review Electronics Letters of June 21, 1979, volume 15, No. 13, pages 377 and 378, how to offset simply and efficiently the parasite capacities C.sub.P1 of the integrated capacities. A constant potential difference must be maintained on the plates of the parasite capacities C.sub.P1 so as to eliminate any charge or discharge of these capacities during operation. For that, a voltage-follower stage is connected between each terminal B.sub.1 of the integrated capacity, which is connected to the plate of the capacity comprising the lower layer of polycrystalline silicon on which C.sub.P1 appears, and a diffused zone of a type opposite that of the substrate which is disposed at the silica-silicon interface facing the polycrystalline silicon layers.